Bio-diesel may be prepared by transesterification of an oil-fat with a monohydric alcohol. Besides fatty acid esters, the products of the transesterification may include monoglycerides, diglycerides, glycerol by-products, as well as the unreacted alcohols and crude oil-fat. Bio-diesel primarily comprises fatty acid esters, and possibly other trace substances such as monoglycerides, diglycerides, glycerol and the like. In the prior art, there are the acid catalysis method, base catalysis method, enzyme catalysis method and supercritical method for the preparation of bio-diesel.
CN1473907A discloses using as raw materials the heels from the refining of vegetable oils and the edible recovered oil, carrying out the production procedures comprising removing impurities by acidification, continuously dehydrating, esterifying, stratifying, and distilling under reduced pressure, and the catalyst used in the process is formed by complex formulation of inorganic and organic acids such as sulfuric acid, hydrochloric acid, p-toluene sulfonic acid, dodecylbenzene sulfonic acid, naphthalene sulfonic acid and the like. The continuous vacuum dehydration is carried out to a water content of less than 0.2% at a pressure of 0.08-0.09 MPa and a temperature of 60-95° C. Additionally, the catalyst is added in an amount of 1-3% in the esterification step; the esterification temperature ranges from 60-80° C.; and the reaction lasts 6 hours. After reaction, the product is neutralized to remove the catalyst, then stratified to remove water, and distilled under reduced pressure to obtain a bio-diesel. The problems of said acid catalysis include slow reaction rate, massive spent acids, and environmental pollution.
DE3444893 discloses a process, wherein an inorganic acid is used as the catalyst; free fatty acids and alcohols are esterified at normal pressure and a temperature of from 50-120° C.; oils are pre-esterified and transesterified in the presence of an alkali metal catalyst. However, the residual inorganic acid catalyst will be neutralized with the alkali, so as to increase the amount of the alkali metal catalyst. Moreover, the pre-esterification will lengthen the processing process, increase the equipment investment, greatly enhance the energy consumption and incur a great loss of the materials. Moreover, the basic catalyst needs to be removed from the product, and a great deal of waste water will be produced.
CN1472280A discloses a process for preparing a bio-diesel, wherein fatty acid esters are used as the acyl receptor, and organisms are catalyzed for interesterification in the presence of a bio-enzyme. However, the presence of an enzyme catalyst has the disadvantages of long reaction time, low efficiency, high enzyme price, and a high possibility of inactivation in high purity methanol.
CN1142993C discloses a process for preparing fatty acid esters by using an oil-fat and alcohol in the absence of a catalyst and under the condition that either of said oil-fat and alcohol is in a supercritical state. The process is carried out using a batch kettle reaction, and is unfavorable to a large industry-scale production
CN1111591C discloses a process for preparing fatty acid esters by continuously reacting an oil-fat with a monohydric alcohol at a temperature of 270 to 280° C. and a pressure of 11-12 Mpa in a tubular reactor. However, the yield of fatty acid methyl ester is only 55-60%.
From the prior art above, it can be found that there are the problems of lower yield of bio-diesel and lower raw material processing capacity in the preparation of a bio-diesel by medium and high pressure methods.